Don't die when one of TANGO_CODESIGNER{,_KEY} is not set.
Don't die when one of TANGO_CODESIGNER{,_KEY} is not set.

When one of TANGO_CODESIGNER{,_KEY} is not set, don't create
{signed,full}_image at all.  Certain follow-on make targets (not run
by 'make build') will fail when this is the case, but there isn't much
we can do about that

Signed-off-by: Dan Nussbaum <86aa56cfef221ac21e80998bdebfb5a7b6754cdd@google.com>

google/tock-on-titan,google/tock-on-titan,google/tock-on-titan